1. Field of The Invention
The present invention relates to optical transmission systems, and particularly to such systems including a transparent sheet having a front side provided with light-control elements which form grooves.
2. Description of The Prior Art
Such a system is manufactured for example by North American Philips Consumer Electronics Corp. and employs a transparent integral sheet having a front and a rear side.
The sheet includes a transparent substrate having front and rear planar surfaces. The front planar surface is provided integrally with at least a pair of mutually parallel elongated light-control elements or ribs. The ribs are laterally spaced to form an elongated "V" or similarly-shaped valley or groove between them. Each rib is a transparent unitary element having two end surfaces and two side surfaces. Specifically, each rib includes an elongated planar back end surface disposed on the substrate and an elongated curved front end surface disposed opposite the back end. The back and front ends are connected by a pair of elongated opposite side surfaces which extend from the back end and are sloped to converge generally in a direction toward the front end. The side surfaces may be planar, and immediately adjacent side surfaces of respective ribs may be contiguous along the substrate front surface. Each rib is dimensioned such that a light ray entering the system through the rear side and traveling in a direction perpendicular to the substrate front surface (operational light ray) can leave the system only through preselected rib surfaces such as the curved front. Therefore, the remaining rib surfaces (e.g. side surfaces) are designed or preselected not to transmit an operational light ray out of the system.
Such systems may be used to display a small lighted image but typically are expanded to include a plurality of usually identical ribs forming respective grooves so that a larger image is displayable. These expanded systems can be incorporated in a rear projection screen and used to display, on an enlarged scale, a television/motion picture image or the like projected by an image source such as a cathode ray tube/film projector and focused within the system's substrate. See for example, U.S. Pat. No. 3,523,717, issued Aug. 11, 1970, entitled "Composite Back Projection Screen" and U.S. Pat. No. 3,830,556, issued Aug. 20, 1974, entitled "Rear Projection Screen."
When so incorporated the system is oriented such that the ribs are elongated in a (vertical) direction parallel to a viewer who is spaced from the front side and who is watching a projected image focused within the system. The curved front surfaces then function refractively to control the light intensity of the image in a (horizontal) direction transverse of the viewer. Thus, the front surfaces may be convex or any other shape which can accomplish the desired transverse control. Light intensity of the image in a direction parallel to the viewer may be controlled by known diffusion means. For a discussion of various surfaces and diffusers for controlling light intensity, see the previously mentioned U.S. patents.
Because such optical systems, whether or not expanded, are often operated in and exposed to visible ambient light, it is desirable to reduce reflection of such light at the front side of the system. Thus, for example, reduction in the contrast of a projected television image focused in the system is lessened.
This reduction in reflection is accomplished by blackening the operationally optically non-transmissive side surfaces of the ribs. Blackening as defined herein includes providing a means for reducing both specular and diffuse reflectivity at the system's front side for all wavelengths in the visible electromagnetic spectrum. In other words, blackening includes providing a means for absorbing visible ambient light.
Known blackening means include a thin (e.g. 50 microns) light-absorbent layer such as a black ink or grease provided on substantially the entire area of the rib side surfaces forming the groove. As discussed in the previously mentioned U.S. patents, this layer is provided intimately on the side surfaces by painting, rolling or other overcoating techniques well known.
Prior art optical transmission systems including the known blackening means have a major drawback because of undesirable absorptive attenuation of operational light by the black layer intimately contacting the relatively large entire area of the rib side surfaces. This attenuation reduces the transmissive efficiency of the system.
As suggested above, these prior art optical systems utilize refractive principles establishing a condition for the phenomenon of total internal reflection to control the direction of operational light traveling through the system. Generally, this condition is established at the rib side surfaces and is not established at the rib front surface.
When a dense medium (e.g. an acrylic plastic) having a first refractive index n.sub.1 interfaces with a second medium (e.g. air) having a second refractive index n.sub.2, it follows from Snell's law that a critical angle .theta..sub.c is determined. Specifically, .theta..sub.c =arcsin (n.sub.2 /n.sub.1) so that a light ray traveling within the plastic medium and incident upon the plastic-air interface at an angle .theta. greater than the critical angle is totally reflected back into the optically more dense plastic.
Optical systems with known blackening means having the light-absorbent layer in intimate physical and, thus, high optical contact with the rib side surfaces forming the groove, in effect, increase the refractive index of the second medium (previously air) cause a corresponding increase in the critical angle. Therefore, operational light transmitted within prior art systems is more prone to unwanted transmission out of such systems through the side surfaces because the range of angles .theta. available for total internal reflection is decreased.